adc: Add adc support based on sysfs

Inpired by IoT.js API, tested on ARTIK710:

```
NODE_PATH=lib node example/adc-example.js
1745
```

Change-Id: Ie9e006c667cbc39747c49251e41a7b25aa9d4607
Signed-off-by: Philippe Coval <p.coval@samsung.com>

Author: rzr

example/adc-example.js

@@ -0,0 +1,47 @@
+// -*- mode: js; js-indent-level:2;  -*-
+// Copyright 2018-present Samsung Electronics France and other contributors
+//{
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//}
+
+var adc = require('adc');
+
+/// ADC0 on ARTIK710 interposer board
+var DEFAULT_ADC_DEVICE = '/sys/bus/platform/devices/c0053000.adc/iio:device0/in_voltage0_raw';
+
+var main = function(config)
+{
+  config = config || {
+    device: DEFAULT_ADC_DEVICE
+  }
+  var port = adc.open(config,  function(err) {
+    if (err) {
+      throw err;
+    }
+    console.log(port.readSync());
+  });
+}
+
+module.exports = main;
+
+if (!module.parent) {
+  var device = process.argv[2] ? Number(process.argv[2]) : DEFAULT_ADC_DEVICE;
+  main({device: device});
+}

lib/adc.js

@@ -0,0 +1,43 @@
+// -*- mode: js; js-indent-level:2;  -*-
+// Copyright 2018-present Samsung Electronics France and other contributors
+//{
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//}
+
+const fs = require('fs');
+
+function Adc()
+{
+  this.open  = function(config, callback) {
+    this.config = config;
+    fs.access(config.device, fs.R_OK, callback);
+    return this;
+  }
+
+  this.readSync = function(err) {
+    var contents = fs.readFileSync(this.config.device, 'ascii');
+    return contents;
+  }
+
+  this.closeSync = function() {
+  }
+}
+
+module.exports = new Adc();